1. Field of the Invention
The present invention relates to a microwave detector, and in particular relates to a double superheterodyne type microwave detector equipped with a first local oscillator and a second local oscillator.
2. Description of the Prior Art
Microwave detectors which generate an alarm upon detecting microwaves having the specific microwave frequencies emitted by radar type speed measurement devices are known in the prior art. Omitting a detailed description of the circuit construction, such microwave detectors generally operate by means of an antenna which picks up microwaves arriving from outside, and a superheterodyne type receiver circuit which receives microwave signals. Further, by sweeping the output of the local oscillator in the related receiver circuit over a prescribed frequency range, such microwave detectors are able to ensure that the reception band width includes the microwave frequency of the detection target. Further, the local oscillator repeatedly carries out only a single sweep of the frequencies within the reception band width for each operation time period.
At this time, the receiver circuit outputs a microwave frequency within the reception band width and two peaks within a prescribed time interval t. Now, because the microwave frequency of the detection target is fixed for the time interval t, the presence or absence of a detection target microwave frequency can be determined by whether or not a pair of peaks are present within the time interval t. In other words, when a signal having the same frequency as the target microwave frequency is received, an alarm is generated.
In this connection, in one type of prior art microwave detector, a fixed frequency oscillator is used in a first local oscillator, and a double superheterodyne type sweeping oscillator is used in a second local oscillator.
In such a prior art microwave detector, because the second local oscillator is made to carry out a sweeping operation, the first intermediate frequency forms a wide band. As a result, there is a widening of the pass band in the intermediate frequency amplifier (band pass filter+amplifier) connected at the step following the first mixer, and this creates the risk that microwave signals near the periphery will leak into such intermediate frequency amplifier.
Namely, the microwave signals that leak into the intermediate frequency amplifier will be amplified as if they were inside the pass band, and then such amplified signals will be inputted into the second mixer. Consequently, in the case where the frequency of such microwave signals matches the output of the first local oscillator for actual target microwaves, it becomes impossible for the circuit arranged after the second mixer to discriminate whether such microwave signals are signals related to actual microwave signals received from the antenna or microwave signals which have leaked into the intermediate frequency amplifier.
Accordingly, when such microwaves leak into the intermediate frequency amplifier, they will be mistakenly interpreted as actual microwave signals, and this will result in the generation of false alarms. In other words, the interference generated by such microwave signals causes the detection accuracy of the microwave detector to be lowered.
Furthermore, this problem is widespread due to the fact that microwaves are emitted by various electronic devices and portable communication devices mounted in the vehicle equipped with the microwave detector or carried into the inside of the compartment thereof. Accordingly, due to the reception of such microwave interference, the generation of false alarms is quite common.